DESCRIPTION (from the application): T cells can be divided into two subsets based on their expression of alphabeta and gammadelta T cell antigen receptors. Since human gammadelta T cells expand in the tissues of patients with certain autoimmune disease, they may be important in autoimmunity. Support for their importance comes from murine models of lupus, autoimmune diabetes, and collagen-induced arthritis, where gammadelta T cells can play regulatory roles limiting autoimmune alphabeta T cell responses. However, the antigens that activate these regulatory gammadelta T cells have not been identified and examples of antigen recognition by gammadelta T cells are few. We recently determined that the predominant human Vgamma2Vdelta2+ T cell subset is stimulated both by the staphylococcal enterotoxin, SEA, and by nonpeptide prenyl pyrophosphate compounds such as isopentenyl pyrophosphate. These prenyl pyrophosphates are essential biosynthetic precursors that are generated both in bacteria and man and thus represent potential autoantigens that could activate regulatory or autoimmune gammadelta T cells. Gammadelta T cell recognition of these nonpeptide antigens was unusual in that it involved a novel extracellular presentation pathway that required cell-cell contact but not antigen processing nor classical antigen presenting molecules. This autoantigen recognition will not be affected by strategies targeted towards altering antigen processing but might be amenable to modulation by altering antigen recognition. Thus, defining the structural elements within the gammadelta T cell receptor will be especially important in identifying strategies to interrupt or stimulate autoantigen recognition by gammadelta T cells to modulate autoimmune diseases. Here, we propose to identify specific TCR residues that mediate recognition by gammadelta T cells of prenyl pyrophosphate antigens and of superantigens using TCR mutagenesis and transfection. In Aim I, we will identify specific gammadelta TCR residues that mediate recognition of the prenyl pyrophosphate antigens by site-directed mutagenesis in complementarity determining regions (CDRs) that we have determined as critical for antigen recognition. In Aim II, we will identify specific Vgamma2 TCR residues that mediate recognition of staphylococcal superantigens by CDR exchanges and site directed mutagenesis. In each case, we will determine if TCR mutations altering recognition of one antigen also alter recognition of the other antigen. These basic studies will provide insights into the mechanisms of antigen recognition by human gammadelta T cells and will help to identify structural elements within the gammadelta T cell receptor that may be targets to alter autoantigen recognition.